#include<stdio.h>
int main()
{
	int x,y; 
	float s;
	scanf("%d",&x);
	scanf("%d",&y);
	s=(x*y)*0.5;
	if((x*y)%2==0)
		printf("%d",(int)s);
	else 
		printf("%0.1f",s); 
	return 0;
}
